a number of elderly patients have low total thyroxine, low free thyroxine index and normal TSH levels. This pattern of thyroid function, not accompanied by profound reductions in serum triiodothyronine concentrations, may represent either true central hypothyroidism or euthyroidism with resetting of the thyroid hormone feedback regulation of TSH secretion. We plan to investigate the pathogenesis of this phenomenon using these patients matched to controls.